1. Field of the Invention
This invention relates to tools, such as wrenches and screwdrivers, for applying a rotational force to a workpiece such as a nut, bolt or screw to loosen or tighten the bolt against a correspondingly threaded part of a stationary object, such as a bolt hole. This invention relates generally to torque wrenches which are used to provide a preselected amount of torque to the bolt, or to measure the amount of torque so provided. In particular, this invention relates to tools for providing or determining the degree of angular rotation applied to the bolt during tightening or loosening.
The torque to be applied to the bolt is often specified by the designer of the equipment being manufactured. A common example of this is the torque specifications for bolts securing the head or an internal combustion engine.
2. Description of the Prior Art
It is well known that specifying the angle of rotation of the bolt, after a predetermined torque has been reached, is a more accurate way of specifying the holding force of the bolt and the stresses involved. In particular, it is known to be desirable to specify the angular rotation to be applied to the bolt in order to reduce the size and strength of the bolt required for a certain value of holding power. Conventional tools for measuring the angle of rotation have been of two basic types.
The first type utilizes an indicator such as a pointer, rigidly mounted to the wrench and an angular scale rigidly affixed to the bolt hole. The bolt is first tightened to a predetermined low-value of torque or to the condition known as "finger tight". The movement of the pointer along the scale thereafter indicates the angle of rotation of the bolt so that the bolt can be tightened to a predetermined angular rotation.
A second type uses a compass-like arrangement mounted to the wrench and a source of magnetic field such as a permanent magnet mounted to the bolt hole. The magnetic field keeps the compass in a fixed orientation with respect to a reference, such as the North Pole, during tightening so that the change in orientation of the "compass" during tightening indicates the angular rotation of the bolt in the bolt hole.
Both of these designs, however, have a major disadvantage in that an object must be positioned in a fixed relationship with the bolt hole. Another disadvantage of such designs are that they are constructed for use in a manufacturing environment, such as on a production line, but are not convenient for use by a mechanic or other operator in a repair facility.